Yeah, you would need a completely new character to re-do the story quests. But a lot of the story stuff is solo play anyway. When she gets to the parts where she needs a group you can help whether you still need to do the quest yourself or whether you finished it ages ago. Personally, id go for the new class(instead of new character) route to be helpful. That way if you run into a situation where you need a bit more power, you can swap to your stronger class and have it ready to go.